Fix issues with supervised user recreation

Fix issues with supervised user recreation.

Issues fixed in this CL:
- Support high-resolution screens when showing the avatar icons.
- Disable import button when there are no supervised users at all to import.
- Don't overwrite the synced avatar on ChromeOS.
- Improve css properties in the import overlay.

A screenshot of the changed import dialog can be found in the bug description of 282464.

BUG=282464, 314605
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=240091

[Adrian Kuegel, 2013-12-10 13:59:10]

Pam, can you please review this CL?

[Pam (message me for reviews), 2013-12-11 10:07:11]

LGTM!

Sorry for the delay. That "also PM for reviews" isn't kidding. :)

- Pam

[Adrian Kuegel, 2013-12-11 10:15:17]

On 2013/12/11 10:07:11, Pam (also PM for reviews) wrote:
> LGTM!
> 
> Sorry for the delay. That "also PM for reviews" isn't kidding. :)
> 
> - Pam

Well, I thought it is not urgent right now, therefore I didn't send a PM. I
could work on the next CL already.
Also, now I found out there were two more locations in the code where we use the
avatar icons which need the imageset statement.
I uploaded a new patch for that, can you please take a look if it still looks ok
to you?

[Pam (message me for reviews), 2013-12-11 10:17:08]

On 2013/12/11 10:15:17, Adrian Kuegel wrote:
> On 2013/12/11 10:07:11, Pam (also PM for reviews) wrote:
> > LGTM!
> > 
> > Sorry for the delay. That "also PM for reviews" isn't kidding. :)
> > 
> > - Pam
> 
> Well, I thought it is not urgent right now, therefore I didn't send a PM. I
> could work on the next CL already.
> Also, now I found out there were two more locations in the code where we use
the
> avatar icons which need the imageset statement.
> I uploaded a new patch for that, can you please take a look if it still looks
ok
> to you?

Still LGTM.

[commit-bot: I haz the power, 2013-12-11 10:17:49]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/akuegel@chromium.org/100073004/50001

[commit-bot: I haz the power, 2013-12-11 12:42:58]

Change committed as 240091